1. Field of the Invention
The present invention relates to a circuit-incorporating photosensitive device using an SOI (Silicon on Insulator) wafer, and especially to a circuit-incorporating photosensitive device which has high sensibility and low power consumption.
2. Description of the Related Art
A circuit-incorporating photosensitive device is widely used as an optical pickup, optical communication, or a photosensor, e.g., a photocoupler. In recent years, there has been intense demand for the higher sensibility, faster operation, and lower power consumption of circuit-incorporating photosensitive devices in all such applications.
FIG. 8 is a cross-sectional view illustrating the structure of a conventional circuit-incorporating photosensitive device 400. The conventional circuit-incorporating photosensitive device 400 shown in FIG. 8 has a laminated structure of a P-type silicon substrate 1 and an N-type silicon substrate 4 epitaxially grown on the P-type silicon substrate 1. In this laminated structure, a photodiode 270, and a bipolar transistor 280 which is a circuit for processing signals output from the photodiode 270 are integrally provided. The N-type silicon substrate 4 is separated into plural regions by P-type embedded diffusion layers 13. The photodiode 270 and the bipolar transistor 280 are respectively provided in the regions separated by the P-type embedded diffusion layers 13.
The photodiode 270 is of a PN junction type, formed with the laminated structure of the P-type silicon substrate 1 and the N-type silicon substrate 4.
The bipolar transistor 280 has a P-type diffusion layer 7 formed in the N-type silicon substrate 4 near the surface thereof. An N-type diffusion layer 8 is formed in the P-type diffusion layer 7. Furthermore, the N-type silicon substrate 4 includes an N-type diffusion layer 6 which extends from the surface of the N-type silicon substrate 4 to an N-type diffusion layer 12.
An oxide film layer 9 is provided on the entire surface of the N-type silicon substrate 4. In the bipolar transistor 280 region, the oxide film layer 9 is provided with wiring 10a connected to the N-type diffusion layer 6, wiring 10b connected to the P-type diffusion layer 7, and wiring 10c connected to the N-type diffusion layer 8 (which is embedded near the surface of the P-type diffusion layer 7).
In the circuit-incorporating photosensitive device 400 having such a structure, the photosensitivity of the photosensitive portion of the photodiode 270 depends on the photosensitivity at the PN junction, as well as the amount of the light absorption corresponding to the size and thickness of the photodiode 270.
In a circuit-incorporating photosensitive device used as an optical pickup, light having a wavelength of about 635 nm for DVD applications, about 780 nm for CD applications, about 850 nm for space optical transmission, or about 950 nm for a photosensor (e.g., a photocoupler) is normally used. The light absorption coefficients of silicon (Si) and light penetration depths into silicon for these wavelengths are shown in Table 1.
As shown in Table 1, the depths to which these light wavelengths penetrate into silicon are no less than 4 xcexcm. Normally, the depths are greater than the thickness of the N-type silicon substrate 4 which forms the circuit-incorporating photosensitive device 400. Thus, the PN junction between the N-type silicon substrate 4 and the P-type silicon substrate 1 is used to improve the photosensitivity of the photodiode 270 and to improve the absorptance at these light wavelengths.
On the other hand, for faster operation and lower power consumption, it is effective to use a SiGe layer (which has a higher light absorptance) as a base layer, as well as an SOI (Silicon on Insulator) wafer, as shown in, for example, Japanese Laid-Open Publication No. 6-61434.
FIG. 9 is a cross-sectional view illustrating a circuit-incorporating photosensitive device 410 in which an SOI wafer 290 is used. The SOI wafer 290 includes a silicon substrate 1 and an N-type silicon substrate 4, with an N-type diffusion layer 3 formed on a lower surface thereof and an oxide film 2 interposed therebetween.
The N-type silicon substrate 4 of the SOI wafer 290 is separated into plural regions by trench-type separation layers 5. A photodiode 270 and a bipolar transistor 280 are respectively provided in the regions separated by the trench-type separation layers 5. The trench-type separation layers 5 extend from the surface of the N-type silicon substrate 4, through the N-type diffusion layer 3, so as to reach the oxide film 2.
In the photodiode 270, a P-type diffusion layer 7a, which serves as an active layer, is formed near the surface of the N-type silicon substrate 4. An N-type diffusion layer 6 is provided so as to extend from the surface of the N-type silicon substrate 4 to the N-type diffusion layer 3.
In an NPN-type bipolar transistor 280 which is a signal processing circuit of the photodiode 270, a base layer 7b formed of SiGe is embedded as a P-type diffusion layer near the surface of the N-type silicon substrate 4. An N-type diffusion layer 8 is provided near the surface of the base layer 7b. Furthermore, in the N-type silicon substrate 4, an N-type diffusion layer 6 is provided so as to extend from the N-type silicon substrate 4 to the N-type diffusion layer 3.
An oxide film 9 is provided on the entire surface of the N-type silicon substrate 4. In the NPN-type bipolar transistor 280 region, the oxide film 9 is provided with an electrode 10a connected to the N-type diffusion layer 6, a base electrode 10b connected to the base layer 7b, and an electrode 10c connected to the N-type diffusion layer 8 (which is embedded near the surface of the base layer 7b).
In the circuit-incorporating photosensitive device 410 having such a structure, the thickness of the silicon layer 7a, which serves as an active layer forming the photosensitive portion of the photodiode 270, is normally about 1 xcexcm, so that there is a problem in that the amount of light absorption is small. Table 1 also shows the light absorptance at different wavelengths in the case where the thickness of the silicon active layer 7a is 1 xcexcm. The light absorptance is 22% for a light wavelength of 650 nm, 11% for a light wavelength of 780 nm, 8% for a light wavelength of 850 nm, and 4% for a light wavelength of 950 nm.
The photodiode 270 has a low photosensitivity because the amount of the light absorption of each of the silicon photosensitive layers 3a, 4a, and 7a is small.
The output of the photodiode 270 having such a low photosensitivity could be subjected to gain compensation by the signal processing circuit. However, when the gain of the output is compensated, the response speed of the signal processing circuit and the signal-to-noise ratio (S/N ratio) may decrease.
According to one aspect of the invention, there is provided a circuit-incorporating photosensitive device comprising: an SOI wafer comprising a first silicon substrate, a second silicon substrate, and an oxide film; a photodiode formed in a first region of the SOI wafer; and a signal processing circuit formed in a second region of the SOI wafer, wherein the photodiode comprises a photosensitive layer comprising an SiGe layer.
In one embodiment of the invention, the photosensitive layer is formed after the signal processing circuit is formed.
In one embodiment of the invention, the photosensitive layer is provided in a recess formed in the SOI wafer.
In one embodiment of the invention, the signal processing circuit comprises a high-speed transistor, and at least a portion of the high-speed transistor is formed of an SiGe layer.
In one embodiment of the invention, the SiGe layer of the photosensitive layer and the SiGe layer of the high-speed transistor are simultaneously formed.
In one embodiment of the invention, the photodiode has a reflection film provided on a bottom surface thereof.
In one embodiment of the invention, the reflection film includes a high melting point metal film.
In one embodiment of the invention, an antireflection film is provided on the photosensitive layer.
In one embodiment of the invention, the antireflection film comprises an SiN film.
In one embodiment of the invention, a thermal oxide film is formed between the photosensitive layer and the SiN layer.
In one embodiment of the invention, the antireflection film is integrally formed of the photosensitive layer.
In one embodiment of the invention, the antireflection film includes an amorphous carbon film.
In one embodiment of the invention, a phase difference between light impinging upon the photosensitive layer and light reflecting at the bottom surface of the second silicon substrate is xc2xd of the wavelength of the light impinging upon the photosensitive layer.
In one embodiment of the invention, the photosensitive layer is separated into plural photosensitive regions by a trench-type separation layer.
In one embodiment of the invention, the photosensitive layer is separated into plural photosensitive regions by forming the photosensitive layer with a selective epitaxial growth method.
Thus, the invention described herein makes possible the advantage of providing a circuit-incorporating photosensitive device with high-sensibility, fast operation, and low power consumption, which prevents a decrease in the S/N ratio.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.